I like it. May need a short description or notification for the end user where they're zaps are going though.
Instead of an error zaps should just go to the user's npub.cash address. Could also be a user setting on my side. What do you think nostr:npub1gcxzte5zlkncx26j68ez60fzkvtkm9e0vrwdcvsjakxf9mu9qewqlfnj5z? 
Discussion
My thinking yes
Also a toggle. On/Off doesn’t make sense on the receiver side (given use case) but on the sender side … “send zaps to ecash if recipient has no lightning address” setting at least keeps users in charge if “black hole sats gone” syndrome occurs.
Coupled with a notification for both parties…